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| United States Patent |
5,091,046
|
|
Hunter
, et al.
|
February 25, 1992
|
Caustic etching of aluminum with matte finish and low waste capability
Abstract
A process for etching aluminum in caustic solution capable of providing a
consistently uniform matte finish like that of the never dump process, but
with little waste like the regeneration process. Etching is performed in a
solution containing free sodium hydroxide and dissolved aluminum in a
ratio between about 0.6 and 2.1 g/l and also containing an etch equalizing
agent at a temperature above about 70.degree. C. Preferably, the etch
solution is regenerated through a crystallization loop.
| Inventors:
|
Hunter; Robert F. (2351 First Street, Burlington, Ontario, CA);
Hohn; Phillip (5 Rannock Street, Toronto, Ontario, CA)
|
| Appl. No.:
|
636458 |
| Filed:
|
December 31, 1990 |
| Current U.S. Class: |
216/93; 216/102; 252/79.5 |
| Intern'l Class: |
B44C 001/22; C23F 001/00 |
| Field of Search: |
156/642,656,665,345
252/79.5
427/309
204/33,38.3,42
134/41
|
References Cited
U.S. Patent Documents
| 2795490 | Jun., 1957 | Newman et al. | 156/665.
|
| 2795491 | Jun., 1957 | Newman et al. | 156/665.
|
| 2975041 | Mar., 1961 | Holman | 156/665.
|
| 4372805 | Feb., 1983 | Takahashi et al. | 156/642.
|
| 4417949 | Nov., 1983 | Lindner et al. | 156/665.
|
| Foreign Patent Documents |
| 1125631 | Jun., 1982 | CA.
| |
Other References
Brown, Craig J., "Regeneration of Caustic Etch Solutions for Aluminum",
Plating and Surface Finishing, Jun. 1982.
Kape, J. M., "Chemical Etching of Aluminium in Caustic Soda Based
Solutions", Transactions of the Institute of Metal Finishing, 1970, vol.
48, p. 43.
Wernick, J.; Pinner, R.; and Sheasby, P.G., "The Surface Treatment and
Finishing of Aluminium and Its Alloys", 1987, pp. 191-203.
Meserve, H. O., Jr., "Chemical Milling Applied to Jet Airliner Body Skins",
Plating, Jan. 1969, p. 44.
|
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Rogers, Bereskin & Parr
Claims
WE CLAIM:
1. A process for etching an aluminum or aluminum alloy work piece to obtain
a desired finish from bright to matte, comprising the steps of:
(a) contacting the work piece with a caustic etch solution under the
following conditions
(i) the etch solution containing free sodium hydroxide and dissolved
aluminum in a ratio of between about 0.6 and 2.1 and also containing an
etch equalizing agent in an effective amount for producing a substantially
uniform etch,
(ii) an etch temperature equal to or higher than about 70.degree. C. and
less than the boiling temperature of the etch solution, and
(iii) an etch time effective for producing the desired degree of etching on
the work piece from a bright finish to a matte finish; and
(b) subsequently separating the work piece from the etch solution.
2. The process of claim 1, wherein the ratio of free sodium hydroxide to
dissolved aluminum in the etch solution is in the range of about 0.8 to
1.9.
3. The process of claim 2, wherein the ratio of free sodium hydroxide to
dissolved aluminum in the etch solution is in the range of about 1.1 to
1.6.
4. The process of claim 1, wherein the concentration of free sodium
hydroxide in the etch solution is in the range of about 10 to 50 g/l.
5. The process of claim 1, 2 or 3, wherein the concentration of free sodium
hydroxide in the etch solution is in the range of about 15 to 45 g/l.
6. The process of claim 4, wherein the concentration of free sodium
hydroxide in the etch solution is in the range of about 20 to 40 g/l.
7. The process of claim 1, wherein the etch temperature is in the range of
about 70.degree. C. to 85.degree. C.
8. The process of claim 4, wherein the etch temperature is in the range of
about 70.degree. C. to 85.degree. C.
9. The process of claim 1, 3 or 6, wherein the etch temperature is about
80.degree. C.
10. The process of claim 1, wherein the etch time is in the range of about
1 minute to 20 minutes.
11. The process of claim 8, wherein the etch time is in the range of about
2 minutes to 11 minutes.
12. The process of claim 3, 4 or 7, wherein the etch time is in the range
of about 2 minutes to 11 minutes.
13. The process of claim 1, 8 or 11, wherein the equalizing agent includes
sodium nitrate.
14. The process of claim 1, 8 or 11, wherein the equalizing agent includes
sodium nitrite.
15. The process of claim 1, 8 or 11, wherein the equalizing agent includes
sodium sulfide.
16. The process of claim 1, 8 or 11, wherein the equalizing agent includes
triethanolamine.
17. The process of claim 1, 8 or 11, wherein the equalizing agent includes
sodium gluconate.
18. The process of claim 1, 8 or 11, wherein the equalizing agent includes
sorbitol.
19. The process of claim 1, 8 or 11, wherein the equalizing agent includes
a combination of sodium nitrate and sodium sulfide.
20. The process of claim 1, further comprising the step of regenerating the
etch solution by separating dissolved aluminum and replenishing sodium
hydroxide, such that the free sodium hydroxide and dissolved aluminum in
the etch bath are maintained at a substantially steady state.
21. The process of claim 20, wherein the regeneration step includes the
steps of removing a portion of the etch solution, separating dissolved
aluminum from said portion, and subsequently returning said portion to the
remainder of the etch solution.
22. The process of claim 21, wherein the step of separating dissolved
aluminum from the removed portion of the etch solution includes the step
of cooling said portion and holding in the presence of seed crystals such
that aluminum hydroxide crystallizes from the solution.
23. The process of claim 1, 8 or 11, wherein the etch solution is
regenerated while repeating steps (a) and (b) by:
(c) removing a portion of the etch solution;
(d) cooling the removed portion and holding in a crystallizer in the
presence of seed crystals such that an aluminum hydroxide containing solid
product is formed, thereby reducing the concentration of dissolved
aluminum and increasing the concentration of free sodium hydroxide in the
removed portion of the etch solution to yield a regenerated caustic
solution having a ratio of free sodium hydroxide to dissolved aluminum
greater than that of the remainder of the etch solution;
(e) separating the regenerated caustic solution from the solid product;
(f) returning the separated, regenerated caustic solution to the remainder
of the etch solution;
steps (c)-(f) being performed in a continual loop such that the etch
solution is maintained at a substantially steady state.
24. The process of claim 1, wherein the step of contacting the work piece
with a caustic etch solution includes immersing the work piece in an etch
bath, and wherein the etch solution is regenerated while repeating steps
(a and (b) by:
(c) removing a portion of the etch solution from the etch bath;
(d) cooling the removed portion to a temperature less than about 65.degree.
C. and holding in a crystallizer in the presence of aluminum hydroxide
seed crystals such that an aluminum hydroxide containing solid product is
crystallized from the removed portion of the etch solution, thereby
reducing the concentration of dissolved aluminum and increasing the
concentration of free sodium hydroxide in the removed portion to yield a
regenerated caustic solution having a ratio of free sodium hydroxide to
dissolved aluminum greater than that of the etch solution in the etch
bath;
(e) separating the regenerated caustic solution from the solid product;
(f) returning the separated regenerated caustic solution to the etch bath;
the concentrations of sodium hydroxide and dissolved aluminum being such
that aluminum hydroxide crystallizes only in the presence of seed crystals
in the crystallizer, and steps (c)-(f) being performed in a continual loop
such that the concentrations of sodium hydroxide and dissolved aluminum of
the etch solution in the etch bath are maintained at a substantially
steady state.
25. The process of claim 20, 21 or 22, wherein the ratio of free sodium
hydroxide to dissolved aluminum in the etch solution is the range of about
0.8 to 1.9.
26. The process of claim 24, wherein the ratio of free sodium hydroxide to
dissolved aluminum in the etch solution is in the range of about 0.8 to
1.9.
27. The process of claim 20, 21 or 22, wherein the concentration of free
sodium hydroxide in the etch solution is in the range of about 10 to 50
g/l.
28. The process of claim 20, 21 or 22, wherein the concentration of free
sodium hydroxide in the etch solution is in the range of about 15 to 45
g/l.
29. The process of claim 24, wherein the concentration of free sodium
hydroxide in the etch solution is in the range of about 15 to 45 g/l.
30. The process of claim 20, 21 or 22, wherein the etch temperature is in
the range of about 70.degree. to 85.degree. C.
31. The process of claim 24, wherein the etch temperature is about
80.degree. C.
32. The process of claim 29, wherein the etch temperature is in the range
of about 70.degree. to 85.degree. C.
33. The process of claim 20, 21 or 22, wherein the etch time is in the
range of about 1 minute to 20 minutes.
34. The process of claim 32, wherein the etched time is in the range of 2
minutes to 11 minutes.
35. The process of claim 26, 29 or 32, wherein the etch time is in the
range of about 2 minutes to 11 minutes.
36. The process of claim 20, 24 or 34, wherein the equalizing agent
includes sodium nitrate at a concentration in the range of about 5 to 20
g/l.
37. The process of claim 20, 24 or 34, wherein the equalizing agent
includes sodium nitrite at a concentration in the range of about 5 to 20
g/l.
38. The process of claim 20, 24 or 34, wherein the equalizing agent
includes sodium sulfide at a concentration in the range of about 0.5 to 6
g/l.
39. The process of claim 20, 24 or 34, wherein the equalizing agent
includes triethanolamine at a concentration in the range of about 5 to 30
g/l.
40. The process of claim 20 or 21, wherein the equalizing agent includes
sodium gluconate.
41. The process of claim 20 or 21, wherein the equalizing agent includes
sorbitol.
42. The process of claim 24, wherein the equalizing agent includes a
combination of sodium nitrate at a concentration in the range of about 5
to 20 g/l and sodium sulfide at a concentration in the range of about 0.5
to 6 g/l.
43. The process of claim 24, 34 or 42, further comprising the step of
recovering the aluminum hydroxide containing solid product after the
regenerated caustic solution is separated therefrom.
44. The process of claim 24, 34 or 42, wherein the etch solution contains
heavy metal sulfides and wherein the process further comprises the step of
substantially separating such heavy metal sulfides from said removed
portion of etch solution prior to cooling said removed portion in a
crystallizer
45. The process of claim 24, 34 or 42, wherein the etch solution contains
heavy metal sulfides and wherein the process further comprises the step of
substantially separating such heavy metal sulfides from said removed
portion of etch solution by filtration.
46. The process of claim 1, 24 or 42, further comprising the step of
rinsing the work piece after separating it from the etch solution, and
wherein a portion of the rinse water is added to the etch solution
substantially to balance evaporation and drag out losses.
47. The process of claim 24, 34 or 42, wherein the etch solution further
comprises an anionic surfactant in an effective amount for producing a
foam blanket on the etch bath.
Description
FIELD OF THE INVENTION
This invention relates to a process for etching aluminum and aluminum
alloys in caustic soda etch solutions.
REVIEW OF RELATED TECHNOLOGY
Work pieces of aluminum and aluminum alloys (hereinafter "aluminum") are
often etched in solutions of caustic soda prior to anodizing. Etching
produces the basic surface finish which will be visible on the anodized
work piece.
During etching, which is conventionally performed at temperatures between
about 55.degree. C. and 60.degree. C., sodium hydroxide reacts with the
aluminum surface to form sodium aluminate, according to the reaction:
2Al+2NaOH+2H.sub.2 O.fwdarw.2NaAlO.sub.2 +3H.sub.2 ( 1)
If this reaction were simply permitted to continue, the level of dissolved
aluminum would rise toward saturation until sodium aluminate would
eventually begin to hydrolize, precipitating aluminum hydroxide and
liberating free caustic soda, according to the reaction:
NaAlO.sub.2 +2H.sub.2 O.fwdarw.Al(OH).sub.3 +NaOH (2)
Under typical etch bath conditions, this precipitate would form a hard
scale on etch tank walls and heating coils which is very difficult to
remove. Simply dumping the etch solution when it reaches aluminum
saturation and replacing it with new solution is both wasteful of
chemicals and hazardous to the environment.
Two alternative processes are in common commercial use to avoid the
precipitation of aluminum hydroxide in the etch tank. In the so called
"never dump process" sequestering agents such a sodium gluconate or
sorbitol are used to stabilize or "tie up" the aluminum and prevent
precipitation. As the aluminum concentration builds up, the etch solution
becomes increasingly viscous. Thus, more of the solution adheres to the
work pieces when they are removed from the etch tank. Ultimately, so
called "drag out" losses of aluminum in the adhering solution balance the
rate of aluminum dissolution from etching.
Effluent treatment of water used to rinse the work pieces after etching and
disposal of the resulting sludge are major disadvantages of this process.
Caustic soda must also be continually added to the etch bath to replace
drag out losses in order to continue the etching reaction. Furthermore,
etch baths of this process require careful temperature control, which is
often difficult, to avoid unwanted precipitation of aluminum hydroxide.
Nevertheless, this process has gained significant commercial acceptance, in
part because it can produce high density random micropitting resulting in
a uniform matte finish. For many applications, a deep matte finish is
preferred to a brighter finish because a matte finish can hide die lines
and scratches better than a bright finish.
The other common commercial process, called the "regeneration process", is
based on precipitating aluminum hydroxide from the etch solution in a
separate chamber and thus preventing precipitation in the etch tank.
Typically, the etch solution is regenerated by running a portion of it
through a crystallizer containing aluminum hydroxide seed crystals. As
aluminum hydroxide is crystallized out, caustic soda is liberated and can
thus be recycled to the etch bath. Since the viscosity of the etch
solution is low, much lower than that of the never dump process, drag out
losses are quite small and only small additions of fresh caustic soda are
needed to balance these losses. Waste treatment is also considerably less
of a problem.
While this process does not have the major waste product problems of the
never dump process, it has unfortunately been found effective to produce
only low intensity micropitting resulting in a fairly bright finish.
Attempts to obtain a matte finish using this process, particularly on work
pieces with significant grain structure such as extrusions, have been
generally unsuccessful. Typically, the finish becomes uneven or
"galvanized".
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a consistent and
easily controlled finish, including a smooth matte finish when desired, by
caustic etching of aluminum with little waste product.
In accordance with the invention, there is provided a process for etching
aluminum to obtain a desired finish, which may range from a bright finish
to a matte finish, comprising the steps of: contacting the aluminum with a
solution containing free sodium hydroxide and dissolved aluminum in a
ratio between about 0.6 and 2.1 and also containing an etch equalizing
agent at a temperature above about 70.degree. C. and long enough to obtain
the desired finish; and subsequently separating the aluminum from the etch
solution.
It has surprisingly been found that certain compounds, when added to etch
solutions similar to those used in the conventional regeneration process,
can reduce or eliminate galvanizing. It has also surprisingly been found
that without employing sequestering agents to tie up aluminum, etching at
a temperature above about 70.degree. C., in combination with levels of
dissolved aluminum generally higher than those used in the conventional
regeneration process, can produce a high quality matte finish like that
obtained by the never dump process.
Preferably, the ratio of free sodium hydroxide to dissolved aluminum in the
etch solution is in the range of about 0.8 to 1.9, and most preferably in
the range of about 1.1 to 1.6. Preferably, the concentration of free
sodium hydroxide in the etch solution is between about 10 and 50 g/l, more
preferably between about 15 and 45 g/l, and most preferably between about
20 and 40 g/l. The etch temperature is preferably between about 70.degree.
C. and 85.degree. C., most preferably about 80.degree. C.
In this specification, the term "equalizing agent" means a compound or
combination of compounds which promote a substantially uniform rate of
etching on the aluminum surface to give a uniform finish. The equalizing
agent of the present invention may include sodium nitrate, sodium nitrite,
sodium sulfide, triethanolamine, sodium gluconate or sorbitol. Preferably,
the equalizing agent includes sodium nitrate, sodium nitrite or sodium
sulfide, and most preferably a combination of sodium nitrate and sodium
sulfide.
Advantageously, the present invention further includes the step of
regenerating the etch solution, preferably by removing a portion of the
etch solution, separating dissolved aluminum from that removed portion,
and subsequently returning the removed portion to the remainder of the
etch solution.
Most preferably, the removed portion is cooled and held in a crystallizer
in the presence of seed crystals such that aluminum hydroxide crystallizes
out from the solution and free caustic soda is liberated. It has been
found that regeneration of the etch solution by means of a crystallization
step provides excellent control of the etch bath chemistry, and thereby
the degree of etching. This process has very low drag out losses and thus
little waste to treat and little make up reagents to add.
The overall advantage is a process which can be easily controlled, and
which produces a consistently even finish that can range from bright to
matte as desired, with little waste product and low reagent costs.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be more clearly understood, reference will
be made to the accompanying drawings in which:
FIG. 1 is a photomicrograph of an etched aluminum surface at 200.times.
magnification, showing a sparkle finish;
FIG. 2 is a photomicrograph of an etched aluminum surface at 200.times.
magnification, showing a smutty finish;
FIG. 3 is a photomicrograph of an etched aluminum surface at 200.times.
magnification, showing a smooth matte finish;
FIG. 4 is a schematic illustration of an etching process according to one
embodiment of the invention;
FIG. 5 is a graph of experimental results showing the effect of dissolved
aluminum level in the etch solution on etch quality;
FIG. 6 is a graph of experimental results showing the effect of total
sodium hydroxide level in the solution on etch rates at various
temperatures;
FIG. 7 is a graph of experimental results showing the effect of sodium
nitrate level in the etch solution on the etch rate;
FIG. 8 is a graph of experimental results showing the effect of sodium
sulfide level in the etch solution on etch rates;
FIG. 9 is a graph of experimental results showing the effect of temperature
on etch rates for two etch solutions.
DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
The process of the present invention requires an equalizing agent in the
etch solution. The equalizing agent reduces or eliminates selective grain
etching, which is known in the trade as "galvanizing". Galvanizing
typically causes a rough sparkle finish, and at times a discontinuous
smutty appearance.
Compounds which are now identified as effective in reducing or eliminating
galvanizing include sodium nitrate, sodium nitrite, sodium sulfide,
triethanolamine, and sodium gluconate. Each of these has been found
effective, in varying degrees, in reducing the sparkle type of galvanizing
shown in FIG. 1 associated with elevated aluminum levels in the etch
solution. The sparkle appearance is caused by very deep or total etching
away of selective grains, forming reflective steps having depths in the
order of about 20 microns. Sodium nitrate was found to be the most
effective of these compounds in reducing or eliminating sparkle type
galvanizing.
Only sodium sulfide was found to be effective in reducing the smutty type
of galvanizing shown in FIG. 2 associated with zinc, which may be present
in some aluminum alloys. The smutty appearance is caused by more intense
micropitting of selective grains, without being generally as deep or
forming reflective steps as found with sparkle type galvanizing. Where the
equalizing agent includes sulfide, there will be a tendency to precipitate
heavy metal sulfides, and if the process includes regeneration of the etch
solution by crystallization, the solution may have to be filtered to
remove such heavy metal sulfides prior to crystallization, since they
could otherwise "blind" the seed crystals in the crystallizer.
The equalizing agent may include a combination of compounds which are
effective in reducing selective grain etching. Triethanolamine, however,
should not be combined with sodium nitrate or sodium nitrite, as this
combination can present a health hazard. The equalizing agent should also
not include sodium gluconate or sorbitol if the process includes
regeneration of the etch solution by crystallization.
It has been found that certain compounds identified as effective equalizing
agents also have other effects. Sodium nitrate to a small degree enters
the etching reaction, dissolving aluminum to form sodium aluminate and
ammonia gas, according to the reaction:
xNaNO.sub.3 +8Al+(8-x)NaOH+(8-2x)H.sub.2 O .fwdarw.8NaAlO.sub.2
+(12-4x)H.sub.2 +xNH.sub.3 (3)
It would be expected from the literature that sodium nitrate would dominate
this etching reaction. Thus, significant levels of sodium nitrate in the
etch solution would be expected to result in the production of large
amounts of ammonia gas. However, at the preferred levels of sodium nitrate
of the invention, the amount of ammonia gas produced has surprisingly been
found to be very small, much less than the amount of hydrogen, and can
easily be vented with typical equipment found on etch tanks used to
operate the conventional regeneration process.
The sodium nitrate reaction also tends to increase the residual free
caustic soda level from what would otherwise be expected, and thus the
process can be operated at higher aluminum concentrations without
precipitating aluminum hydroxide. This can be advantageous when the
process includes regeneration of the etch solution by crystallization,
because crystallization is generally more efficient at higher aluminum
concentrations. It would be expected in fact that significant levels of
sodium nitrate would result in a build up of excess sodium hydroxide.
However, it has surprisingly been found that at the preferred levels of
sodium nitrate of the invention, the excess sodium hydroxide liberated is
low enough just to balance drag out losses.
Sodium sulfide, as well as reducing selective grain etching which causes
both sparkle and smutty type galvanizing, also reduces overall etch rates.
This can be advantageous since the high etching temperatures of the
present invention tend to have relatively fast etch rates. These can be
difficult to accommodate in a commercial process due to limitations in
manipulation of the work pieces for short residence times.
The conventional regeneration process is typically operated at a
temperature between about 55.degree. C. and 60.degree. C., with an
aluminum concentration between about 25 and 30 g/l, and a free sodium
hydroxide between about 50 and 70 g/l for aluminum. In the present
invention, the etch temperature is higher than about 70.degree. C.,
preferably about 80.degree. C. While the temperature of the etch solution
could be as high as its boiling point, at temperatures much above about
85.degree. C. the rate of etch becomes inconveniently fast for commercial
applications. The etch reaction is exothermic, and it has been found that
operation at about 80.degree. C. facilitates temperature control.
Etching at such high temperatures would be expected to result in so called
"transfer stains", that is, streaking of the surface which can occur as a
work piece is transferred to a rinse operation after etching. However, it
has surprisingly been found that with the etch solutions of the present
invention, no transfer stains result during typical commercial transfer
times despite the use of etch temperatures above about 70.degree. C.
The ratio of free sodium hydroxide to dissolved aluminum is between about
0.6 and 2.1, and preferably between about 1.1 and 1.6. In the conventional
regeneration process, the levels of free sodium hydroxide and dissolved
aluminum are typically in a ratio greater than 2:1.
It has been found that the elevated etch temperature and higher aluminum
concentration are effective in producing a desirable matte finish, as
shown by the microstructure of FIG. 3. Such a matte finish is created by
even, random micropits having a frequency in the order of 3000-4000
pits/mm.sup.2 and an average depth of about 5 or 6 microns, effectively
obliterating grain boundaries. Micropitting for a bright finish would
typically have a frequency of only about 500 pits/mm.sup.2 and an average
depth of about 2 microns. Too low an aluminum concentration will leave
residual brightness even at an elevated temperature. However, too high an
aluminum concentration will initiate white spotting and streaking. Thus, a
fairly well defined aluminum concentration range is required at a
temperature above about 70.degree. C. to obtain a high quality matte
finish. It has been found that regeneration of the etch solution by
crystallization provides a highly satisfactory means for control of the
aluminum concentration.
The degree of matte finish obtained at the elevated temperature and within
the optimum aluminum concentration range will also depend upon the etch
time, which controls the amount of aluminum removed from the surface of
the work piece. The elevated etching temperatures of the present invention
can raise etch rates to a point where appropriate residence times to
produce the desired finish could be rather short for convenient crane
manipulation of the work piece. Also, the high gassing rate of hydrogen
and caustic mist might cause air quality problems and overflow swelling of
the etch bath.
In the present invention, the elevated etch rates which would otherwise be
caused by the high etching temperatures are lowered to more conventional
ranges by reducing the total caustic concentration. Additionally, the use
of small amounts of sodium sulfide can lower the etch rate significantly.
Sodium sulfide in concentrations as low as 1 g/l can lower the etch rate
by about 25%.
A foaming surfactant may also be employed to create a foam blanket on the
etch bath surface, and adhering to the work pieces when they are withdrawn
from the etch tank for rinsing. The foam entraps caustic mist and thereby
improves air quality, and also reduces heat losses from the etch bath
surface at the elevated temperatures.
After etching, the work pieces would typically be rinsed with water.
Although the amount of drag out losses of caustic soda and other etch
solution chemicals are low, due both to the low visocity and the low
concentration levels of the etch solution, the present invention permits
some of these losses to be conveniently recovered, and thus reduced even
further. At the elevated etch temperatures of the present invention,
evaporation of water from the solution can be significant. Water
evaporation can be made up from recycled rinse water, thereby reducing
chemical losses and also reducing waste treatment.
Without being bound by theory, it is believed that the temperature and
concentrations of dissolved aluminum and free sodium hydroxide of the etch
solution in the present invention promote the formation of a very thin and
porous film of aluminum hydroxide at the interface between the solution
and the surface of the work piece. The etch solution attacks the surface
through random micropores in this film, leading to intense, evenly
distributed micropitting of the surface that is apparent as a matte
finish. The high temperature promotes the reaction kinetics so that the
aluminum hydroxide film is maintained and does not dissipate away. It is
further believed that the equalizing agent evens the thickness and
porosity of the aluminum hydroxide film, and thus counteracts the effects
of alloy segregation and grain orientation that lead to galvanizing. Thus,
the equalizing agent tends to equilibrate the rate of etch and promotes a
uniform surface finish.
The present invention, in its broadest scope, could be practised on the
basis of dumping and replacing the etch solution when the aluminum
concentration becomes too high. However, this would not meet the objective
of a low waste product. It is preferred to operate the invention with
regeneration of the etch solution to maintain a substantially steady
state. Regeneration by ion exchange, dialysis, or other techniques may be
effective. It is most preferred though to regenerate the etch solution by
continually passing a portion thereof through a crystallizer.
Crystallization removes dissolved aluminum as recoverable aluminum
hydroxide crystals, and maintains the aluminum level in the etch solution
in the appropriate range. Crystallization can also remove trace levels of
heavy metal contaminants by co-crystallization. This can aid in reducing
smutty type galvanizing. Crystallization furthermore liberates free sodium
hydroxide for the etching process. Where the equalizing agent includes
sodium nitrate, additional sodium hydroxide is liberated during
crystallization, which can make up for drag out losses.
According to a preferred embodiment of the invention as illustrated in FIG.
4, etching on an architectural anodizing line is performed batchwise. An
etch tank 10 contains an etch bath 12 with a foam blanket 11. Extruded
aluminum alloy work pieces 14 are cleaned, placed on a rack 13, and then
immersed through the foam blanket 11 into the etch bath 12.
The etch bath 12 is a caustic soda solution having about 60 g/l of total
sodium hydroxide. Free sodium hydroxide is about 27 g/l, and dissolved
aluminum is about 25 g/l. The solution also includes about 12 g/l of
sodium nitrate and about 1 g/l of sodium sulfide. The foam blanket 11 is
produced by the addition of a foaming surfactant sold under the trademark
DOWFAX 2A-1, at a concentration of 0.022 cc./l.
The etch bath 12 is maintained at a temperature of about 80.degree. C. by
means of heating coils in the etch tank 10.
The etch is allowed to proceed for the time required to produce the desired
degree of etching on the work piece, from a bright finish to a matte
finish. For extrusions, an etch time of about 5 minutes has been found
effective to produce a smooth matte finish.
After etching, the rack 13 is lifted and the work pieces are allowed to
drain for about 10 to 20 seconds, following which they are transported by
crane to a rinse tank. After rinsing, the work pieces may be acid
de-smutted and anodized in a conventional manner.
During etching, a portion of the solution from the etch bath 12 is
continually removed and directed to a crystallizer 21, at a rate which is
adjusted to maintain a substantially steady state in the etch bath,
depending on the surface areas of the work pieces 14, the throughput, and
the degree of etch. For example, to produce a matte finish on extruded
work pieces at a rate of 100 m.sup.2 /hr requires continuous regeneration
at a rate of approximately 16 l/min.
The removed portion of the etch solution is first passed through a
pre-crystallizer filter 20 to remove heavy metal sulfides. The filtered
solution is then introduced to the crystallizer 21, which is maintained at
about 55.degree. C. by means of a water jacket. The cooling water exiting
the water jacket is used for rinsing.
The etch solution enters the crystallizer 21 with about 25 g/l of dissolved
aluminum and about 27 g/l of free sodium hydroxide. In the crystallizer
21, aluminum hydroxide crystallizes from the solution on aluminum
hydroxide seed crystals. Aluminum hydroxide crystals are continually
filtered and removed in a crystallizer filter 22. The aluminum hydroxide
recovered from the crystallizer filter 22 can be sold, for example, for
use in producing alum. As aluminum hydroxide crystallizes, sodium
hydroxide is liberated. Regenerated etch solution is returned from the
crystallizer 21 to the etch bath 12 at the same rate at which solution is
removed from the etch bath 12 and introduced to the crystallizer 21. The
regenerated solution contains about 15 g/l of dissolved aluminum and about
42 g/l of free caustic soda. This maintains the etch bath 12 at a steady
state and at the required concentrations of dissolved aluminum and free
caustic soda to attain the desired matte finish.
Sodium nitrate is the basic make up chemical. This must be added in an
amount of about 0.2 g of sodium nitrate per gram of aluminum dissolved.
Nitrate in the solution results in excess caustic liberation in the
crystallizer 21. This should approximately balance drag out losses of
caustic soda, although slight adjustments with either small amounts of
nitric acid or small amounts of sodium hydroxide may be required.
The off gas consists largely of hydrogen, with small amounts of ammonia.
Gas evolution can entrain caustic solution as a mist. The foam blanket 11
effectively removes the caustic mist from the off gas.
The invention will now be further illustrated by the following examples
which demonstrate the operability and preferred conditions of the process,
but which in no way limit the scope of the invention.
EXAMPLE 1
This example shows the effect of a variety of additives in a etch solution
at conventional operating temperatures, between 55.degree. C. and
60.degree. C.
Etch baths were prepared generally as "reacted" baths, with the aluminum
used to set the initial aluminum concentration being dissolved in a
pre-mixed caustic plus additive solution. In this way, a simulation of
steady state conditions, with any additive by-products, could better be
achieved. The bath was then analyzed by acid titration for total caustic,
free caustic and aluminum.
Test pieces of 6063 aluminum alloy extrusions were cleaned for two minutes
at 50.degree. C. in a conventional anodizing line cleaner, rinsed, and
then immersed on PVC coated wire or PVC plastic racks in a 0.7 l etch bath
for a designated time. Temperature was controlled to within +/-1.degree.
C. After etching, the pieces were withdrawn from the bath, held in air for
45 seconds to simulate transfer time, rinsed in cold water, de-smutted in
either 12% sulphuric or 12% nitric acid for 5 minutes, rinsed again and
then dried.
Test pieces were evaluated with respect to surface finish both visually and
microscopically. Microscopically, the frequency of random micropits, pit
depth, grain boundary etching, and grain to grain differences
(galvanizing) were ascertained. Samples were also assessed for stain and
macropitting frequency and severity. The overall etch depth or amount of
aluminum removed was determined by either weight change or micrometer
measurements.
The results are summarized in Table I. From these results it will be noted
that of the 19 additives, only sodium nitrate, sodium nitrite, sodium
sulfide, sodium gluconate and triethanolamine reduce or eliminate sparkle
type galvanizing, and only sodium sulfide reduces or eliminates smutty
type galvanizing. None of the additives resulted in a smooth matte surface
at an etching temperature of 55.degree. to 60.degree. C.
EXAMPLE 2
This example shows the effects of etch temperature and dissolved aluminum
and sodium hydroxide concentrations, with additions of sodium nitrate and
sodium sulfide.
Tests were conducted generally as described for Example 1, but in a 150 l
etch tank. Total caustic soda concentrations of 60-65 g/l and 105 g/l were
tested with varying ratios of aluminum and free caustic soda. Temperatures
from 58.degree. C. to 85.degree. C. were tested. The results are shown in
Table II. These results show a progressive increase in the degree of matte
finish between 70.degree. C. and 83.degree. C. A temperature of at least
about 70.degree. C. is required to obtain a satisfactory degree of random
microetching to produce a smooth matte finish. The results also show that
aluminum concentration at the elevated temperatures has a significant
effect on the intensity of the matte finish. Too low an aluminum
concentration produces a brighter finish. Too high an aluminum
concentration produces visible macropits, namely white spots and flecks.
This condition is reduced as the temperature is increased.
For any specific total caustic concentration, there is a fairly well
defined optimum range for the aluminum concentration to produce a smooth
matte finish at temperatures above 70.degree. C. For example, for a total
caustic concentration of 60 g/l, the preferred aluminum concentration is
between about 20 and 28 g/l. Such a range of aluminum concentration is
compatible with the control range of a tied crystallization regeneration
loop.
TABLE I
__________________________________________________________________________
Effect of various additives on finish at 55.degree. C.-60.degree. C.
Galvanizing
condition**
Additive
General (i.e., Bright Sparkle
Additive Concentration
Type of Finish and/or
Contamination
Test Type Range g/l
Etch Finish*
Non-Uniform Smut)
Build-up
__________________________________________________________________________
Series 1-1
Sodium Nitrate
3.0-40 Bright, smooth
Eliminate sparkle type
None
Series 2-1
Sodium Nitrite
3.0-20 Bright, smooth
Eliminate sparkle type
None
Series 3-1
Sodium Sulfide
1.0-10 Bright, smooth
Eliminate sparkle and
Sulfate from air
oxidation
Series 4-1
Sodium Sulate
10-25 Bright No effect None
Series 5-1
Sodium Chloride
10-25 Bright No effect None
Series 6-1
Sodium Fluoride
1 Bright No effect
Series 7-1
Sodium Molybdate
1.0-10 Bright, but modest
No effect
smoothing re die lines
Series 8-1
Sodium Phosphate
10-30 Bright, but modest
No effect
smoothing re die lines
Series 9-1
Sodium Gluconate
10 Bright Some reduction in
sparkle type
Series 10-1
Sodium Tartrate
10 Bright No effect
Series 11-1
Sodium EDTA
10-40 Bright No effect
Series 12-1
Triethanolamine
5.0-30 Bright, smooth
Eliminate sparkle type
Series 13-1
Dowfax 2A-1
0.25-1.0
Bright No effect Foam scum
Series 14-1
Dowfax 3B-2
0.25-1.0
Bright No effect Foam scum
Series 15-1
Zinc Sulfate
0.15-0.50
Dull, non-uniform smut
Creates smut type
Excessive black smut
Series 16-1
Tin Sulfate
0.3 Bright No effect Excessive black smut
Series 17-1
Nickel Chloride
1 Bright No effect
Series 18-1
Copper Chloride
1 Bright No effect Excessive black smut
Series 19-1
Ammonium 5.0-10 Bright Causes deep Sulfate
Persulfate macro pits
__________________________________________________________________________
*Etch Bath NaOHT = 40-120 g/l, Al = 10 g/l Higher than crystallization
final concentration, 55-60.degree. C., 1.0-1.5 mil removal.
**Selective Grain Etch
The results also demonstrate that the amount of aluminum removed from the
surface of the work piece has a significant effect on the finish, but only
at temperatures greater than about 70.degree. C. This degree of control is
advantageous for processing a range of alloys that may have different
etching responses.
EXAMPLE 3
This example shows the effect of different concentrations of two compounds
which may be included in an equalizing agent, namely sodium nitrate and
sodium sulfide.
Bench scale tests were employed using a procedure as described for Example
1.
The results are shown in Table III, and in the graph of FIG. 5. These
results show that sparkle type galvanizing is caused by elevated aluminum
concentrations, and is overcome by sodium nitrate at concentrations above
about 5 g/l. The effectiveness of sodium nitrate in reducing or
eliminating sparkle type galvanizing is apparent both at conventional
temperatures of 50.degree. C., and at the higher temperatures of the
present invention. Sodium nitrate is not effective in overcoming smutty
type galvanizing. Sodium sulfide does however reduce or eliminate smutty
type galvanizing and is effective in concentrations as low as 1 g/l (using
a 60% sodium sulfide hydrated commercial reagent). Sodium sulfide can also
reduce sparkle type galvanizing, but is not as effective in reducing
sparkle type galvanizing as sodium nitrate. Sodium nitrate and sodium
sulfide can be used together in order to reduce or eliminate both sparkle
type and smutty type galvanizing in concentrations as low as about 5 to 8
g/l sodium nitrate and 0.6 to 1 g/l sodium sulfide.
TABLE II
__________________________________________________________________________
Effects of temperature and concentrations of aluminum and hydroxide on
finish with equalizing agent
__________________________________________________________________________
Laboratory Free NaOH
Test Run No.
Total NaOH g/l
g/l Al g/l
Temp. .degree.C.
Al Removed Mils
NaNO3 g/l
Na2S* g/l
Dowfax 2A-1
__________________________________________________________________________
cc/l
1-2 105.0 75.0 20.0
58.0 1.2 20.0 10.0 10.0
2-2 105.0 75.0 20.0
70.0 1.2 20.0 10.0 10.0
3-2 105.0 56.0 33.0
70.0 1.2 20.0 10.0 10.0
4-2 105.0 75.0 20.0
83.0 1.2 20.0 10.0 10.0
5-2 105.0 56.0 33.0
83.0 1.2 20.0 10.0 10.0
6-2 105.0 56.0 33.0
83.0 1.4 20.0 10.0 10.0
7-2 105.0 56.0 33.0
83.0 1.7 20.0 0.0 10.0
8-2 105.0 56.0 33.0
83.0 0.9 20.0 0.0 10.0
9-2 105.0 56.0 33.0
83.0 0.7 20.0 10.0 10.0
10-2 60.0 29.0 21.0
70.0 1.5 12.0 1.0 0.022
11-2 65.0 21.0 30.0
70.0 1.5 12.0 1.0 0.022
12-2 60.0 29.0 21.0
75.0 1.5 12.0 1.0 0.022
13-2 65.0 21.0 30.0
75.0 1.5 12.0 1.0 0.022
14-2 65.0 24.0 28.0
80.0 1.5 12.0 1.0 0.022
15-2 65.0 24.0 28.0
85.0 1.5 12.0 1.0 0.022
__________________________________________________________________________
ETCH QUALITY
Laboratory
Degree of
Test Matte Die line Macro Stain after
Run No.
Finish
hiding power
pitting de-smut
Micro-structure
__________________________________________________________________________
1-2 Zero Zero None None Grain boundary
2-2 Fair Fair None None Grain boundary + modest random micro
etch
3-2 Good Good None None Grain boundary + modest random micro
etch
4-2 Good Good None None Grain boundary + modest random micro
etch
5-2 Excellent
Excellent None None Minimum grain boundary: general
random micro etch
6-2 Excellent
Excellent None None Minimum grain boundary: general
random micro etch
7-2 Excellent
Excellent None None Minimum grain boundary: general
random micro etch
8-2 Good Good None None Grain boundary + medium random micro
etch
9-2 Good+
Good+ None None Modest grain boundary + medium
random micro etch
10-2 Good Good None None Modest grain boundary + medium
random micro etch
11-2 Good+
Some die line pits
Severe None Modest grain boundary + medium
random micro etch
12-2 Good+
Good+ None None Modest grain boundary + medium
random micro etch
13-2 Excellent
Some die line pits
Some pitting
None Minimum grain boundary: general
random micro etch
14-2 Excellent
Modest die line pits
Modest pitting
None Minimum grain boundary: general
random micro etch
15-2 Excellent
Excellent No pitting
None Minimum grain boundary: general
random micro etch
__________________________________________________________________________
*commercial hydrated sodium sulfide reagent approx. 60% by weight Na2S
EXAMPLE 4
This example shows the effects of different concentrations of total sodium
hydroxide, sodium nitrate and sodium sulfide on etch rates at various
temperatures.
Tests were conducted generally as described for Example 1, although some
tests were performed in a 150 l etch tank and using aluminum extrusion
work pieces having a surface area approximately four times the surface
area of the etch bath to simulate typical commercial etching operations.
The results are shown in Table IV, and in the graphs of FIGS. 6-9. These
results show that reducing the total caustic concentration by one half
reduces the etch rate by approximately one half. Small additions of sodium
sulfide also further reduce the etch rate of less concentrated caustic
solutions by about 25%. Sodium nitrate has little effect on etch rate. The
combination of a total sodium hydroxide concentration of about 60 g/l with
1 g/l of sodium sulfide provides a bath activity at temperatures between
about 75.degree. C. and 80.degree. C. only slightly higher than that
obtained at 60.degree. C. in an etching solution having a total sodium
hydroxide concentration of about 120 g/l as would typically be used in the
conventional regeneration process.
The use of a surfactant produced a foam blanket on the etch bath surface
which would adhere to the work pieces after their removal from the etch
bath and during transfer, significantly reducing evolution of caustic
mist. The surfactant employed in these tests was of the anionic
diphenyloxide disulfonate type, as manufactured by the Dow Chemical Co.,
of Midland, Mich., and sold under the trademark DOWFAX 2A-1.
EXAMPLE 5
This example shows the operability of the etching process of the present
invention, and resulting surface finish after anodizing.
Etching was conducted in a 450 l etch bath placed adjacent to a commercial
anodizing line. Samples were etched in accordance with the invention in a
solution containing 64.1 g/l of total sodium hydroxide, 35.2 g/l free
sodium hydroxide, 21.6 g/l dissolved aluminum, 12.0 g/l sodium nitrate,
1.0 g/l sodium sulfide and 0.022 g/l DOWFAX 2A-1. The etch temperature was
80.degree. C. and the etch time was 5 minutes, giving approximately 40
microns average metal removal. After etching, the samples were rinsed and
acid de-smutted in the usual manner, and then run through the anodizing
and sealing operations of the commercial anodizing line.
TABLE III
__________________________________________________________________________
Effect of concentrations of sodium nitrate and sodium sulfide on
__________________________________________________________________________
finish
Surfactant
Laboratory
Total
Free Etch Al Concentrat.
Test NaOH
NaOH
Al Bath Removed
cc/l Dowfax
NaNO3
Na2S
Zn
Run No.
g/l g/l g/l Temp. .degree.C.
Mils 2A-1 g/l g/l ppm
__________________________________________________________________________
1-3 120.0
60-90
20-40
55-60 1.2-2.5
0.0 0.0 0.0
2-3 120.0
68.0
35.0
60.0 1.9 0.0 >5.0 0.0
3-3 120.0
84.0
20.0
60.0 2.0 0.0 20.0 0.0 112.0
4-3 120.0
84.0
20.0
60.0 1.5 0.0 20.0 10.0
112.0
5-3 105.0
75.0
20.0
60.0 1.5-2.0
0.0 20-30
0.0 <30
6-3 101.0
48.0
38.0
83.0 1.7 0.0 10.0 0.0 2.0
7-3 101.0
45.0
38.0
83.0 1.0 3.0 10-15
0.0 2.0
8-3 101.0
45.0
38.0
83.0 1.0 3.0 10-15
.gtoreq.1.0
2.0
9-3 60.0
28.0
21.0
80.0 1.5 0.0 0.0 0.0 <2
10-3 60.0
30.0
20.0
80.0 2.5 0.01 0.0 1.0 <2
11-3 60.0
23.0
25.0
80.0 1.7 0.01 0.0 4.0 <2
12-3 60.0
23.0
25.0
80.0 1.7 0.01 12.0 4.0 <2
13-3 60.0
23-33
18-25
80.0 1.8 0.01 12.0 1.0 <2
14-3 61.0
36.0
18.7
80.0 2.0 0.01 12.0 0.0 <2
__________________________________________________________________________
Laboratory
Test Galvanizing Intensity
Run No.
and Description
__________________________________________________________________________
1-3 Sparkle type with selective grain "step" etch
2-3 Bright, smooth, no galvanizing
3-3 Non-uniform, smut type galvanizing. Air sparge streaking.
4-3 Bright, smooth, no galvanizing
5-3 Bright, smooth, no galvanizing
6-3 Matte-satin, desirable finish, no galvanizing
7-3 Severe, non-uniform smut type galvanizing with selective
grain micro etch
8-3 Matte-satin, desirable finish, no galvanizing
9-3 White cast but sparkling and not satin
10-3 Fairly matte, but areas of sparkle grain
"step" galvanizing
11-3 Matte with fair satin smoothness. Borderline of
acceptability.
12-3 Matte-satin, desirable finish; no galvanizing
13-3 Matte-satin, desirable finish; no galvanizing
14-3 Matte-satin, desirable finish; no galvanizing
__________________________________________________________________________
The samples were then visually evaluated. The surface finish of the
anodized samples was consistently excellent, with a uniform matte
appearance like that of anodized work pieces etched by the conventional
never dump process.
EXAMPLE 6
This example shows the operability of regenerating etch solutions of the
present invention by crystallization.
Etch baths were prepared generally as described for Example 1 and batch
type crystallization tests were carried out in 1.5 l stainless steel
vessels. The tests were performed at 50.degree. C. for 24 hour periods
using 15% by volume seed crystals of non-washed aluminum hydroxide
obtained from a commercial crystallization system. An etch solution having
a composition typical of that used for the conventional regeneration
process, with a total sodium hydroxide concentration of 105 g/l and no
added sodium nitrate, sodium sulfide or other equalizing agents, was used
as a control.
Crystallization tests were performed with and without pre-filtration. In
tests where a high concentration of foaming surfactant was added, a
crystal settling aid was added. The settling aid was an anionic
polyelectrolyte sold under the trademark ALCHEM 81C09-SC at a
concentration of 1 ppm in the etch solution.
TABLE IV
__________________________________________________________________________
Effect of total caustic concentration on etch rates
__________________________________________________________________________
ETCH BATH CONDITION
Total
Free Al Dowfax
Etch
Lab test
NaOH
NaOH
Al Temp
Time
Remv.
NaNO3
Na2S
2A-1 Rate
run No.
g/l g/l g/l .degree.C.
Min.
Mils
g/l g/l g/l g Al/m2/min
__________________________________________________________________________
1-4 0-120
68-90
20-35
50-85 0.0 0.0 0.0 See FIG. 6
2-4 120.0
68.0
35.0
60.0 0-40 0.0 0.0 See FIG. 7
3-4 120.0
68.0
35.0
60.0 0-40 0-10
0.0 See FIG. 8
4-4 61.0
36.0
19-22
70-80
5-11
1.5 12.0 0-1.0
0.022
See FIG. 9
5-4 105.0
64.0
28.0
60.0
8.6
1.5 0.0 0.0 0.0 12.0
6-4 105.0
65.0
27.0
85.0
3.2
1.5 12.0 0.0 0.0 32.0
7-4 60.0
16-29
21-30
70-80
5-14
1.5 12.0 1.0 0.022
7-20
__________________________________________________________________________
WORK PIECE TRANSFER
GASSING CHARACTERISTICS OF BATH
CHARACTERISTIC
Surface area Stain
of Al Qualitative
Foam
Fume Fume during
Lab test Surface area
gassing
height
during
during transfer
run No. of tank
rate cm. etch transfer
(after de-smut)
__________________________________________________________________________
1-4
2-4
3-4
4-4
5-4 4/1 Normal,
Zero
Moderate
Moderate
Zero
controlled
6-4 4/1 Vigorous
2.5 Severe
Severe Zero
not (Froth)
controllable
7-4 4/1 Normal,
5.0 Low Moderate
Zero
controlled
(Fine except for
foam) water vapour
__________________________________________________________________________
The results are shown in Table V. These results show that crystallization
rates are not significantly altered by the presence of sodium nitrate,
sodium sulfide, or the foaming surfactant, or by the use of a lower total
sodium hydroxide concentration than typically used in the conventional
regeneration process. Commercial crystallization rates are attainable with
the etch solutions of the present invention, and the concentrations of
aluminum and free sodium hydroxide in the etch bath can be adjusted by the
alteration of the solution flow rate through the crystallization loop.
For example, a lower flow rate through the crystallization loop will cause
the dissolved aluminum in the etch bath to increase and the free sodium
hydroxide to decrease, while the aluminum removal in crystallization will
increase until a new steady state is reached. Where the etch solution
includes sodium sulfide, filtration prior to crystallization can remove
heavy metal sulfides which could otherwise blind the seed crystals in the
crystallizer and thus reduce crystallization rates. Where the etch
solution includes sodium nitrate, dissolution of aluminum during etching
uses slightly less sodium hydroxide than is liberated during
crystallization. The additional liberated sodium hydroxide can make up for
drag out losses.
Many modifications of the preferred embodiments described above in detail
can be made within the broad scope of the present invention.
TABLE V
__________________________________________________________________________
Effects of crystallization on etch solution aluminum and free hydroxide
concentrations
__________________________________________________________________________
ETCH BATH CONDITION
NaOH
Total
Free Al Dowfax
Alchem
Consumed
Type of
NaOH
NaOH
Al Temp Time
Remv.
NaNO3
Na2S
2A-1 81 Co9-Sc
Al
TEST oper.
g/l g/l g/l .degree.C.
Min.
Mils
g/l g/l g/l ppm dissolved
__________________________________________________________________________
Conventional
Lab 105.0
61.2
28.7
60.0 9.4 1.5 0.0 0.0 0.0 0.0 1.5
Regeneration
Process
1-6
Range
2-6 Line 105.0-
60.0-
25.0-
55.0-
7.0-
1.0-
0.0 0.0 0.0 0.0 1.5
130.0
80.0
35.0
60.0 11.0
1.5
Matte-satin
Lab 61.0
34.1
20.1
80.0 5.0 1.5 12.0 1.0 0.022
0.0 1.3
finish
regeneration
process
3-6 105.0
57.4
33.8
80.0 3.3 1.5 14.0 1.0 1.0 1.0 1.4
__________________________________________________________________________
CRYSTALLIZER CONDITIONS
Not NaOH
Estimated
Total Free .DELTA.
NaOH Created
Chemical
NaOH NaOH Al Free
.DELTA.
Produced
g NaOH/g
Make-up
Etch Anodized
Initial
Final
Initial
Final
Initial
Final
NaOH
Al Al Al Crys-
g/gAl Surface
Surface
TEST g/l g/l g/l g/l
g/l g/l
g/l g/l
crystal.
tallized
dissol.
Quality
Quality
__________________________________________________________________________
Conventional
105.0
103.6
61.2
72.8
28.7
20.8
11.6
7.9
1.5 0.0 0.26 Bright
Shiny
Regeneration NaOH sparkle
obvious
Process severe
die lines
1-6 die lines
2-6 14.8
10.0
1.5 0.0 0.23- Bright
Shiny.
0.35 Sparkle
Obvious
NaOH severe
die lines.
die
Non-s.
Regions
uniform
of
with dull
uniform
smut
smut.
areas.
Matte-satin
61.0
64.3
34.1
45.3
20.1
12.7
11.2
7.4
1.5 0.2 0.03 Matte-
finish * * * * NaOH satin
regeneration 20.1
15.5
6.8 4.6 0.36 minimal
process NaNO3 die lines
3-6 106.0
109.1
57.4
78.2
33.8
19.8
20.7
13.9
1.5 0.1 0.12 Matte-
NaOH satin
0.17 minimal
NaNO3 die
__________________________________________________________________________
lines
*Indicates not prefiltered
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